The ionosphere, embedded in and tightly coupled to the thermosphere, is strongly influenced by couplings to other geophysical regions. For example, above it, both the magnetosphere and plasmasphere greatly affect the ionosphere by the precipitation of soft and energetic particles, by heat conduction, and by fluxes of thermal particles. Below, the middle atmosphere affects it with upwardly propagating waves (gravity waves, tides, and planetary waves). All the while, polar and auroral regions greatly affect the mid-latitudes by the equatorward penetration of electric fields and winds, and by the equatorward propagation of waves (traveling ionospheric disturbances or TIDs).
Exploring these couplings effectively furthers our understanding of at least the dominant processes and interactions that play such an important role in determining the character of this part of the Earths environment. Significant progress during the Solar-Terrestrial Energy Program (STEP) has demanded that the observational, analytical, and theoretical thrusts of the international scientific community be global in all senses. Observationally, this has led to coordinated measurements from many regions, from the poles to the equator, and from ground- and space-based instruments. It has also led to many different instruments, including new ones, measuring an extensive variety of (related) geophysical parameters. Depending on the instrument, measurements have been made continuously or at appropriate intervals to sample different geomagnetic conditions, and diurnal, seasonal, inter-annual, and solar-cycle variations. Extensive analyses have been carried out on these observations. New empirical models have been developed and old ones improved. Theoretical work has led to new and improved first-principles models that are being used to test our understanding of the observations. Our intent is to review this progress and suggest some future directions. Our approach is to illustrate the broad front of progress with representative examples, rather than to do an exhaustive review. We apologize from the start, to those whose good work we have not noted, in order to allow a broad balanced overview of areas advanced.